The purposes of this study were to evaluate tridiphane {2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane} as a postemergent herbicide with atrazine {2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine} and cyanazine {2-((4-chloro-6-(ethylamino)-s-triazin-2-yl)amino)-2-methylpropionitrile} for annual grass control in corn (Zea mays L.). In addition, the behavior of tridiphane in plants and its possible role in modifying the herbicidal behavior of atrazine was examined.Field studies indicated that tridiphane was a highly active postemergent herbicide on both grasses and broadleaf weeds. Corn, however, proved to be very tolerant to tridiphane-triazine combinations, with only slight phytotoxicity being exhibited by cyanazine treatments. Tridiphane-triazine combinations proved much more effective against grasses than did the triazines alone. This was especially true when grasses were larger than 5 cm in height. Giant foxtail (Setaria faberii Herrm.) was the grass species under study in field plot evaluations.The behavior of tridiphane was examined by foliar application of ('14)C-tridiphane to corn and German millet (Setaria italica L.). Approximately two-thirds of the ('14)C-tridiphane applied was lost through apparent volatile escape from the leaf surface. The majority of the remainder resided in the area of application. Only very small amounts were translocated to other plant parts. No ('14)C-tridiphane was found in the roots. Application of ('14)C-tridiphane in the presence of atrazine did not alter these findings in either species.Foliar applications of ('14)C-atrazine were made to both species with and without tridiphane. Corn treated with tridiphane absorbed about twice as much ('14)C-atrazine as corn not receiving tridiphane treatment. Absorption by German millet was not affected by tridiphane. Tridiphane did not affect the xylem mobility of ('14)C-atrazine in either species.Tridiphane's most significant effect on atrazine behavior was its effect on metabolism. The enzymatic detoxification of atrazine by glutathione-s-transferase was greatly diminished in corn by tridiphane. Free atrazine levels did not, however, rise appreciably. Instead, the free atrazine was hydroxylated to the extent that free atrazine levels remained approximately constant with or without tridiphane. In German millet, atrazine levels rose slightly as glutathione-conjugated atrazine levels fell. Hydroxylation did increase, but not as much as in corn.